Some image-forming devices well known in the art have a function for correcting registration errors, that is, irregularities in positions at which images are formed on paper. This type of image-forming device first forms a registration pattern or other pattern configured of a plurality of marks on a conveying belt or the like. The image-forming device detects the positions of the marks on the belt with photosensors to determine positional relationships among the marks. Based on these positional relationships, the image-forming device can correct irregularities in image-forming positions.
However, the detection signal received from a photosensor sometimes includes signal waves associated with light reflected off scratches, stains, foreign matter, and the like (hereinafter referred to simply as “scratches and the like”) on the belt, for example. Hence, it is possible that the image-forming device will erroneously detect these scratches and the like as marks of the pattern, reducing the accuracy with which the image-forming device can detect the positional relationships of marks.
In light of this problem, one conventional image-forming device disclosed in Japanese unexamined patent application publication No. 2003-98795 attempts to remove signal waves associated with scratches and the like (hereinafter referred to as “noise waves”) from the detection signal so as to extract only the signal waves corresponding to marks (hereinafter referred to as “mark waves”) by comparing the wave height of each signal wave in the detection signal to a threshold. This image-forming device is also capable of modifying the threshold value.